1. Field of the Invention
The present invention relates to an apparatus for suppressing electric field radiation from a cathode ray tube used in a Visual Display Terminal (hereinafter referred to VDT) in a computer system.
2. Description of the Background Art
It has been recognized that the electric field radiation from a VDT, particularly that in the range from 5 Hz to 400 kHz adversely affects human health. According to the MPR standards (MPR 1990) in Sweden, for example, alternating electric fields (AEF) are regulated so as not to exceed 25 V/m and 2.5 V/m in the ranges from 5 Hz to 2 kHz and 2 kHz to 400 kHz, respectively. Regarding TCO standards, electric fields are regulated so as not to exceed 10 V/m and 1 V/m in the ranges from 5 Hz to 2 kHz and 2 kHz to 400 kHz, respectively.
Electric fields radiated from a VDT are largely caused by the flyback pulses of horizontal deflection, and such electric fields are generated mainly from a deflection yoke mounted on the cathode ray tube. The electric fields caused by flyback pulses of horizontal deflection are within a relatively high frequency domain as described above. Other than those caused by the flyback pulses of horizontal deflection as described above, the VDT also radiates electric fields caused by high voltages applied to the anode of the cathode ray tube. These voltages fluctuate in synchronization with the vertical field. The electric fields caused by voltage fluctuations in the vertical field are observed in a relatively low frequency domain in the above-described frequency range.
Such electric field radiation from portions other than the front surface of the cathode ray tube can be effectively shielded by a shielding device such as a metal plate, for example. At the front of the cathode ray tube, where images are visually displayed, such shielding by the metal plate cannot be simply applied.
Meanwhile, in order to prevent the front surface of the cathode ray tube from becoming charged due to the high voltage striking the cathode ray tube surface and thus attracting dust from the air, a transparent conductive film is formed either on the front surface of the cathode ray tube or on a transparent panel bonded to the front of the cathode ray tube and grounded. Such a film has a shielding effect to some extent, but applying the film can be costly.
U.S. Pat. No. 5,260,626, for example, discloses an apparatus for suppressing such electric field radiation of horizontal components. Using the apparatus, a pulse obtained from a horizontal transformer or the like (such as flyback transformer and deflection yoke) is inverted in polarity, shaped and amplified into a waveform very close to the waveform of radiation but of opposite polarity. Regarding the vertical component, a pulse is also obtained by resistor-diving the anode voltage. This pulse is also shaped and amplified such that its shape is very close to that if the waveform of vertical radiation but is again of opposite polarity. The radiation of electric fields (both horizontal and vertical) is then suppressed by applying these opposite polarity pulses to a degaussing coil mounted at the front of the cathode ray tube.
According to another method described in U.S. Pat. No. 5,151,635, a sensor, such as an electrode plate, is attached in proximity to a cathode ray tube. Electric fields radiated from the cathode ray tube are directly detected by the sensor. The detection signal is inverted, then amplified and supplied to an antenna mounted around the cathode ray tube which again causes suppression of the electric field radiation.
Among the two aforementioned methods, the first method sufficiently satisfies MPR-II according to the AEF standards, but more precise detection waveforms for cancellation are required in order to satisfy TCO standards. More specifically, the first method of canceling electric fields by resistor-dividing the anode voltage requires that the detection resistance be several hundred M.OMEGA. or higher, which degrades the frequency characteristic and causes difficultly in accurately producing display picture components. Furthermore, noises within the flyback transformer may be detected as well.
Regarding the second method, positional deviation errors of either the sensor of radiation from the antenna, will impede the electric fields from being accurately detected, and canceled. The setup needed for this method is also complicated.